This was due to unhealthy eating, smoking, drinking and lack of physical activity, the study said.

It expressed concern over the proportion of overweight and obese children, which had increased from 17 percent in the last report card, in 2007, to 20 percent last year.

"Being overweight and inactive increases the risk of a host of life-threatening diseases, including diabetes and cardiovascular disease," the study said.

Only 42 percent of young people participated in sufficient vigorous physical activity, down from 45 percent in 2007.

Hours dedicated to television or computers stood at 30 percent, up from 25 percent in 2007.

Less than 70 percent of high school learners had regularly scheduled physical education classes, and the rates were lowest among schools in poor communities.

Unhealthy eating was also a major concern, the study said.

Nearly 30 percent of teens consumed fast food two to three times a week.

One of the major barriers to healthy eating was affordability of healthy food, which cost almost twice as much as the unhealthy equivalent.

The prevalence of smoking among adolescents had remained constant since 2007, despite a hike in the age at which young people could legally buy cigarettes from 16 to 18.

Smoking was more common among boys than girls, and in urban areas.

One in five teenagers surveyed admitted to being current smokers. This was well above the global prevalence for children and youth.

Although there were signs of greater awareness and an increasing number of intervention programmes by government such as introducing physical education into the curriculum, and encouraging schools to have vegetable gardens, this had yet to yield results, study co-author Professor Vicki Lambert said.

The Healthy Active Kids SA Report Card 2010 drew on over 95 published studies and reports.

The previous report was for 2007. Sapa

Are South African children heading in the same direction as British and American? Leave a comment below or write to editor@thesouthafrican.com